The invention relates primarily to a CMOS image sensor, which, in contrast to a CCD sensor, can be addressed directly, like a customary random access memory. In a CMOS sensor, fluctuations in the technological parameters lead, in addition to a 1/f noise of the individual pixels, to a noise (fixed pattern noise, FPN) essentially corresponding to a fixed image pattern. This noise is caused by production-dictated fluctuations in the threshold voltage of the transistors (offset FPN), the gain between the read-out path (gain FPN) and the leakage currents in the sensor cells (dark FPN).
The offset noise of an individual pixel can be suppressed for example by multiple sampling and correlation of the values read out (correlated double sampling, CDS). In order to correct the offset FPN, it is possible, for example, to carry out a subtraction with a reference image recorded with the diaphragm closed, the reference image being recorded anew after each image or, alternatively, only being recorded once. If such a reference image is recorded each time, the maximum possible image rate is greatly reduced, and if the reference image is only recorded once, dark current correction is possible only with difficulty. A further possibility is to read an always darkened line and subtract it from the content of the line memory, thereby resulting in a saving of memory but only partial correction of the dark current. In high end systems, for example, a gain FPN correction is subsequently carried out with digital signal processing, the correction being carried out by pixel-by-pixel multiplication of signal by stored correction coefficient and requiring a very high outlay on hardware and memory.
The object on which the invention is based consists, then, in specifying a method and a device for the exposure-dependent noise correction in image sensors which can be addressed in lines and columns which enables maximum quality of the correction of the FNP noise in conjunction with a low outlay.
This object is achieved according to the invention by means of the features of patent claim 1 with regard to the method, and by means of the features of patent claim 6 with regard to the device. Further claims relate to advantageous refinements of the method according to the invention and of the device according to the invention.
Particular advantages of the invention reside in the programmability of the correction parameters and in the fact that the FNP correction can be coordinated with the illumination conditions and the correction components can be switched on and off individually.